This invention relates to separation of relatively light components in a feed mixture from relatively heavy components using a stripping medium. The invention is especially suited for separating lighter hydrocarbons and undesirable components from heavier hydrocarbons in a petroleum refinery feedstock.
Product stripping to remove lower boiling range components from much higher boiling range components of a petroleum refinery feedstock is well known. Removal of such lower boiling range components is typically desired to enable the lower boiling range refinery product to meet certain product flash specifications required by the petroleum products market. A product flash specification is generally stated in terms of the temperature that will cause the sample to produce enough vapor to ignite when exposed to an open flame. Also, product stripping is used to remove hydrogen, hydrogen sulfide, and ammonia in petroleum feedstocks after hydrotreating.
The typical stripping technique employs a column or tower that provides contact between a feedstock liquid flowing downward in the column and a stripping vapor that flows upward through the column. The stripping vapor is most commonly steam from an external source, but the vapor can alternatively be produced within the bottom of the column by boiling the bottoms product using an external heat source and equipment commonly referred to in the industry as a reboiler.
Use of a reboiler may have the disadvantage of causing high temperatures which result in undesired feedstock product cracking. Also, hydrogen sulfide from the feedstock may accumulate in high concentrations in the reboiler and result in troublesome corrosion.
The disadvantage of using steam for stripping is that it saturates the stripped product with water. Often, the stripped product must then be dried or the water otherwise removed for the product to meet market specifications. Heating oil, for example, should not contain significant quantities of water.
In refineries, steam stripping is most commonly used in olefin gas-oil hydrotreating. However, an olefin gas-oil hydrotreater, designed for stripping with steam, cannot be used for treating heating oil without addition of a reboiler and an external heat source for stripping without steam, or a product drier for removing water left behind in the product by steam stripping.
Occasionally, an olefin gas-oil hydrotreater may be needed to provide additional capacity for treating heating oil or to provide a substitute for a heating oil hydrotreater shut down for repair. At such times, the disadvantages of steam stripping according to the design of the olefin gas-oil hydrotreater are readily apparent.
There continues to be a need for an efficient, economical process for stripping petroleum feedstocks, and particularly heating oil, of light components without contaminating the stripped product with water, and without expensive and time consuming equipment modifications.